This relates generally to DC-DC converters, and more particularly to DC-DC converters using a flying capacitor in a low voltage and high voltage operating environment.
FIG. 1A schematically shows an example of an automotive power delivery path in a normal operation mode 100. As shown in FIG. 1A, a 12 volt battery 102, an alternator and rectifier 104, and a DC-DC power converter 106 are connected in parallel to a bus 108, such that the alternator and rectifier 104 charges the battery 102. The converter 106 shapes the power signal delivered to the other devices 110 on the vehicle. The converter 106 will typically receive a power signal between 8 and 16 volts (generally, up to 20 volts) and convert it to, e.g., 3.3 volts or 5 volts.
FIG. 1B schematically shows an example of an automotive power delivery path in a load dump operation mode 112. Disconnection of the vehicle battery 102 from the alternator (and rectifier) 104 while the battery 102 is being charged is called a “load dump.” Vehicle battery 102 disconnection can be caused by various conditions, such as power cable corrosion, poor connections within the power circuit, or an intentional battery disconnect while the vehicle is running. When a terminal of the battery 102 is disconnected, inductive current from the alternator and rectifier 104 continues to be provided to the bus 108. Magnitude of current supplied by an alternator is controlled by the current in the field winding. Load dumps result in voltage surges that are typically brief (e.g., 40 ms to 400 ms), comprising inductive current through the bus 108 at significantly more than 8 to 16 volts; generally, up to 40 volts. The voltage surge is caused by the alternator's 104 regulator being unable to decrease the field current fast enough to prevent the current provided by the alternator 104 from causing the voltage on the bus 108 to significantly increase. Generally, the converter 106 comprises switches (typically transistors) that are rated to handle the maximum 40 volt signal that can be received during a load dump voltage surge.
FIG. 2 shows an example graph of voltage against time 200 for a voltage received by a converter 106 from an automotive battery 102. As shown in FIG. 2, while voltage of an automotive power signal 202 (pre-converter 106) mostly stays well under 20 volts 204, that voltage can—generally rarely, e.g., 0.1% of usage lifetime—spike as high as 40 volts 206. Generally, as further described with respect to FIG. 5E, lower-voltage-rated transistors are more efficient, can be switched faster, take up less device area, and can have better figures of merit (better figures of merit generally relating to lower impedances, lower impedances correlating with increased efficiency) than higher-voltage-rated transistors. This means that for 99.9% of their usage lifetime, 40 volt transistors in a converter 106 are over-rated for their purpose.
FIG. 3 schematically shows an example buck converter 300. Generally, a buck converter is a DC-DC power converter which steps down voltage and steps up current from the converter's input (supply) to its output (load). In the buck converter 300 as shown in FIG. 3, an input voltage Vin 302 is connected to the drain of a first transistor 304. The source of the first transistor 304 is connected to the drain of a second transistor 306 and a first terminal of an inductor 308. The source of the second transistor 306 is connected to ground GND 310. The gate of the first transistor 304 is biased by a control signal Q1 312, and the gate of the second transistor 306 is biased by a control signal Q2 314. A capacitor 316 and a resistor 318 are connected in parallel between a second terminal of the inductor 308 and GND 310. The voltage at the second terminal of the inductor 308 comprises an output voltage Vout 320 of the converter 300.
The first and second transistors 304, 306 are generally controlled such that a transistor turns on (is activated to become conductive) after the other turns off (to prevent a short from Vin 302 to GND 310). Accordingly, only one of the two transistors 304, 306 is on at a given time. Consequently, the first transistor 304 and the second transistor 306 will be required to withstand the full input voltage Vin 302: when the first transistor 304 is on, the second transistor 306 will have voltage Vin 302 between its drain and source, and when the second transistor 306 is on, the first transistor 304 will have voltage Vin 302 between its drain and source.
When the first transistor 304 is on, current flows from Vin 302 across the inductor 308. Current through the inductor 308 is described by
      V    =          L      ⁢              dI        dt              ,where V and I are voltage across and current through the inductor 308, respectively, and L is inductance of the inductor 308. During this period, the voltage across the inductor is V=Vin−Vout, because the first transistor 304 connects the inductor 308 to Vin 302. Because a buck converter steps voltage down across an inductor, Vin is greater than Vout. Therefore, V is positive, current increases, and energy is delivered across the inductor 308 when the first transistor 304 is on.
When the second transistor 306 is on, current flows from GND 310 across the inductor 308. During this period, the voltage across the inductor 308 is V=−Vout, because the second transistor 306 connects the inductor 308 to GND 310. Therefore, V is negative and current decreases, ramping down energy (V*I) delivered across the inductor 308 when the second transistor 306 is on.
The integral of voltage across the inductor 308 over time should generally equal (or converge to) zero to prevent current through the inductor 308 from rising without limit; this is the zero voltage condition. The duty cycles of the first and second transistors 304, 306 are generally selected to satisfy this condition. Specifying D as the duty cycle of the first transistor 304 and 1−D as the duty cycle of the second transistor 306, the zero voltage condition can be expressed as:D*(Vin−Vout)+(1−D)*(−Vout)=0
This simplifies to
  D  =      Vout    Vin  and Vout=D*Vin. Because power delivered by the input side equals power received by the output side (ignoring losses due to, for example, resistive elements), Vin*Iin=Vout*Iout, where Iin is the input current and Iout is the output current. This further shows that, when the zero voltage condition is satisfied, Iin=D*Iout.